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vSphere Host Power Management (HPM) is a technique that saves energy by placing certain parts of a computer system or device into a reduced power state when the system or device is inactive or does not need to run at maximum speed. The term host power management is not the same as vSphere Distributed Power Management (DPM) [1], which redistributes virtual machines among physical hosts in a cluster to enable some hosts to be powered off completely. Host power management saves energy on hosts that are powered on. It can be used either alone or in combination with DPM.

vSphere handles power management by utilizing Advanced Configuration and Power Interface (ACPI) performance and power states. In VMware vSphere® 5.0, the default power management policy was based on dynamic voltage and frequency scaling (DVFS). This technology utilizes the processor’s performance states and allows some power to be saved by running the processor at a lower frequency and voltage. However, beginning in VMware vSphere 5.5, the default HPM policy uses deep halt states (C-states) in addition to DVFS to significantly increase power savings over previous releases while still maintaining good performance.

HPM Power Policy Options in ESXi 5.5
ESXi 5.5 offers four different power policies that are based on using the processor’s ACPI performance states, also known as P-states, and the processor’s ACPI power states, also known as C-states. P-states can be used to save power when the workloads running on the system do not require full CPU capacity. C-states can help save energy only when CPUs have significant idle time; for example, when the CPU is waiting for an I/O to complete. ESXi 5.5 offers the following power policy options:
• High Performance: This power policy maximizes performance, using no power management features. It keeps CPUs in the highest P-state at all times. It uses only the top two C-states (running and halted), not any of the deep states (for example, C3 and C6). High performance is the default power policy for ESXi releases prior to 5.0.
• Balanced: This power policy is designed to reduce host power consumption while having little or no impact on performance. This is the default power policy since version 5.0. ESXi has used P-states in the Balanced power policy since 5.0. Beginning in ESXi 5.5, we now also use deep C-states (greater than C1) in the Balanced power policy. Formerly, when a CPU was idle, it would always enter C1. Now ESXi chooses a suitable deep C-state depending on its estimate of when the CPU will next need to wake up.
• Low Power: This power policy is designed to save substantially more power than the Balanced policy by making the P-state and C-state selection algorithms more aggressive, at the risk of reduced performance.
• Custom: This power policy starts out the same as Balanced, but it allows individual parameters to be modified.